Generally, electronic devices, such as portable radios, are becoming physically smaller and customers and manufacturers are demanding more features. Consequently, some radios require a compact integrated antenna to provide either a second antenna for diversity or to conceal the primary antenna for cosmetic purposes.
Since most of the surface area of a portable radio is normally obstructed by a user's hand, a logical location for an integrated antenna is in an extended portion of the radiotelephone housing. This extended housing may be realized by rotating a keypad cover outwards, by twisting a portion of the radiotelephone housing, or by sliding a portion of the radiotelephone housing from a first position to a second position. Such a portable radio has valid modes of operation when the housing element is in the first position as well as in the second position.
Consequently, any antenna or RF circuit designed to be integrated into a movable housing element must be designed such that it performs well in both in the first position and the second position. A difficulty in the antenna design arises when the antenna in the second position is in close proximity to the electrical components of the portable radio and the antenna in the first position is further away from the inner components of the radio. Typically, an antenna must be tuned to match the impedance of the power amplifier for maximum performance of the antenna. The matching of an antenna is highly dependent upon the position of the antenna during its operation. Here, the antenna has two physical positions in which it must operate efficiently. If the antenna is tuned when in the first position, then when the antenna is in the second position, near the electrical components of the transceiver, the antenna is detuned. A detuned antenna has a poor impedance match to the power amplifier and suffers a substantial loss of performance. Thus, it is necessary to develop an antenna that functions efficiently when the movable housing element is in the first position and in the second position.